Anatomy · Embryology and Genetics

Down syndrome

2 min read

Down syndrome (trisomy 21): numerical chromosomal disorder with an extra copy of chromosome 21, karyotype 47,XX,+21 or 47,XY,+21.
Most common numerical chromosomal abnormality in humans, occurring in about 1 in 700 births.

Types of Trisomy 21

Differences between triplo-21 and translocation mongolism

Point	Triplo-21 (nonfamilial mongolism)	Translocation mongolism (familial mongolism)
Chromosome number	47, with an extra separate chromosome 21	46, extra chromosome 21 attached to another autosome
Mechanism	Nondisjunction of the 21st pair of chromosomes at meiosis	Translocation of chromosome 21 onto another autosome
Inheritance pattern	Nonfamilial	Familial
Frequency among cases	Majority of cases	About 3% of cases

Point

Chromosome number

Triplo-21 (nonfamilial mongolism): 47, with an extra separate chromosome 21
Translocation mongolism (familial mongolism): 46, extra chromosome 21 attached to another autosome

Mechanism

Triplo-21 (nonfamilial mongolism): Nondisjunction of the 21st pair of chromosomes at meiosis
Translocation mongolism (familial mongolism): Translocation of chromosome 21 onto another autosome

Inheritance pattern

Triplo-21 (nonfamilial mongolism): Nonfamilial
Translocation mongolism (familial mongolism): Familial

Frequency among cases

Triplo-21 (nonfamilial mongolism): Majority of cases
Translocation mongolism (familial mongolism): About 3% of cases